Field of the Invention
The present invention mainly relates to an organic electro-luminescent (EL) display.
Discussion of the Background Art
A panel unit of an organic EL display having a top emission structure typically has a configuration in which an organic EL element substrate (a Thin-Film transistor: TFT substrate) and a color filter substrate are bonded together.
An organic EL element substrate having a conventional structure includes, for example, a glass substrate, a TFT structure, planarizing resin, an inorganic passivation film that may be optionally provided, an underlying layer for enhancing adhesiveness, reflective electrodes, an insulating film that has openings in locations serving as light emitting parts, an organic EL layer, a transparent electrode (including semitransparent one), and a barrier layer that covers a structure including and below the transparent electrode within a display area, in this order. The transparent electrode is connected to wiring in a circumferential edge part of the organic EL element substrate.
On the other hand, a color filter substrate having a conventional structure includes, for example, a glass substrate, black matrix, color filters, and color conversion layers that may be optionally provided, in this order. As a method for forming the color filters and/or color conversion layers, in addition to a conventional photolithography method, an application (coating) method such as an inkjet method is also becoming widely used. In the case of using the application method to form plural kinds of color filters and/or color conversion layers, it is common to use a separation wall to selectively form each of the color filters and/or color conversion layers in a desired location.
Finally, an organic EL display is obtained by bonding the organic EL element substrate and the color filter substrate together while opposing the light emitting parts of the organic EL element substrate against the color filters of the color filter substrate with registering. The bonding can be performed by a method such as a vacuum dripping bonding method that is commonly used for liquid crystal display manufacturing. In order to highly efficiently transmit light from the organic EL element substrate side to the color filter substrate side and keep mechanical strength, filler such as an adhesive may be enclosed in a bonding gap. If accurate control of the bonding gap between the organic EL element substrate and the color filter substrate is desired, spacers can be provided on the color filter, or the like. If the bonding gap is too wide, a crosstalk problem that light penetrates into an adjacent pixel is concerned. On the other hand, if the bonding gap is too narrow, influence of interference, mechanical contact between the organic EL element substrate and the color filter substrate in the display area, and the like are concerned. In particular, in the case of enclosing the filler in the bonding gap, if the bonding gap is too narrow, the included filler may distribute unevenly.
For example, in the case of forming the color conversion layer by an application method using a bank, the color conversion layer is formed by forming a separation wall having a desired shape and applying color conversion layer-forming ink with use of an inkjet device or the like in the state that a processing surface, on which the separation wall has been formed, faces upward, to form the color conversion layer. The color conversion layer-forming ink is generally prepared by dissolving or dispersing a color converting material into a solvent. An ink droplet just after the application has a raised shape to the extent that it protrudes from an upper part of the separation wall. Then, by heat-drying the ink droplet, a planar layer is formed in the bottom of the separation wall. Usually, by repeating the application and heat-drying a plurality of times, the color conversion layer having a desired film thickness is obtained.
In this case, the separation wall is required to have a height of about 3 to 7 μm in order to prevent an ink droplet from leaking at the time of the application. Such a height cannot be ignored as compared with a sub pixel size of about 40 to 60 μm in a display having a resolution of 200 to 150 ppi, and thereby the efficiency may be reduced by increase of a ratio of emitted light that does not enter the color conversion layer of a predetermined sub pixel but deviates transversely. In addition, in the case where the transversely deviated light enters an adjacent different color sub pixel, the color conversion layer of the adjacent sub pixel may emit converted light and the emitted light may produce undesired hue.
Further, the thin-film organic EL layer has extremely low mechanical strength. Therefore in the organic EL display employing the bonding structure as described above, mechanical bonding strength in the display area cannot be expected at all. For this reason, concern on film peeling due to heat shock or impact applied when used, shrinkage by curing at the time of solidifying the enclosed filler, or the like is inherent in a large-screen bonding-type organic EL display, even if a whole of the display area is bonded by filling the adhesive into the bonding gap. Also, a structure involving a hollow bonding gap has been proposed by many institutions from the start time of the organic EL display; however, it goes without saying that such a structure is not advantageous to manufacturing of the large screen display.
In addition, in the top emission type organic EL display in which the organic EL layer is formed by a vapor deposition, the barrier layer for protecting the organic EL layer is substantially essential. However, in the case where the structure in which the filler is enclosed in the bonding gap is adopted and the filler has a lower refractive index than that of the barrier layer, total reflection occurs at an interface between the barrier layer and the filler, preventing sufficient amount of light from being transmitted, and therefore resulting in an optical loss. For example, SiN or the like that is commonly used for the barrier layer has a refractive index of about 1.7 to 1.9. On the other hand, the types of resins having a higher refractive index than this are very few, and therefore a range of filler selection is narrowed to constrain a manufacturing method. In addition, such filler is a special and expensive material, and therefore becomes a factor for increasing manufacturing cost. Meanwhile, in the case of forming the barrier layer with use of a material such as SiO having a refractive index of about 1.5 in order to conform the refractive index to that of typical resin, reflection at the interface between the barrier layer and the filler is reduced, but a large loss occurs in light on the way from the organic EL layer and transparent electrode to the barrier layer. This is because the organic EL layer and the transparent electrode typically have reflective indices of about 2, respectively.
As a method for solving the optical loss caused by the filler enclosed in the bonding gap as described above, Japanese Patent Laid-Open No. 2006-32010 proposes a structure in which a forward tapered shaped insulating film for separating a cathode is used as a separation wall on which an organic EL layer, transparent anode, and protective layer are formed, and on the protective layer, a color filter and/or color conversion layer is formed (see PTL 1). In this structure, the insulating film used as a bank and the organic EL layer of a light emitting part are joined to each other throughout a whole screen. Here, the insulating film that is formed of an organic material and extremely thick as compared with the other layers contains a large amount of outgas such as moisture. An amount of outgas transmitted through the insulating film increases because the film thickness is large. In a state where the insulating film and the organic EL layer are joined to each other throughout the whole screen, the outgas is easily transmitted from the insulating film to the organic EL layer, which may cause an area called as a dark spot or dark area to be spread in a short period of time. Further, Japanese Patent Laid-Open No. 2006-32010 does not disclose sealing based on substrate bonding; however, the sealing is practically required. Therefore, as described above, there is a concern on insufficient mechanical strength due to the thin film organic EL layer.
In addition, it is necessary to consider the transmission of the outgas or solvent through the protective layer. Even if taking countermeasures against the insulating film used as the bank (more stringent heat treatment condition at the time of forming the insulating film, or the formation of the insulating film with an inorganic material), continuity of the organic EL layer over the whole display area, per se, may cause broad area transmission of the outgas that penetrates through the protective layer. In particular, in the case of applying a color converting material on the protective layer to form the color conversion layer by a wet process such as an inkjet method, the organic EL layer is significantly damaged by a component penetrating through the protective layer. This is because it is necessary to dilute the color converting material down to 1 to a few % with an organic solvent that significantly damages the organic EL layer in the formation of the color conversion layer by the wet process. This situation is tantamount to applying the solvent on the organic EL layer and protective layer, therefore dramatically increasing a possibility that the component damaging the organic EL layer penetrates from defects such as pinholes, which are sparsely present in the protective layer, to give rise to dark areas in a wide region of the display area.
On the other hand, Japanese Patent Laid-Open No. 2008-78038 or the like proposes an organic EL element substrate in which a separation wall is used to separate an organic EL layer and the like into plural parts (see PTL2 or the like). A structure in Japanese Patent Laid-Open No. 2008-78038 is intended to use a vapor deposition method having high directionality to separate the organic EL layer with use of the separation wall. At the same time, the structure is intended to expose an auxiliary electrode formed on the substrate and to form a transparent electrode by a sputtering method exhibiting relatively good coverage (i.e., having low directionality) to connect the transparent electrode to the auxiliary electrode. Besides this proposal, it is well known that, an orthogonal grid shaped separation wall is formed on the organic EL element substrate side in the case of using inkjet or the like to form the organic EL layer itself. Alternatively, for example, if a metal transparent electrode that is thin enough to transmit light is formed by a vapor deposition method having high directionality as done in the organic EL layer, and the transparent electrode is separated into a plurality of lines by the separation wall, it become possible not to control the display area as a whole, but to control it for each of the lines. Accordingly, in view of versatility of control, forming the separation wall on the organic EL element substrate side itself could be easily thought. However, in such an organic EL display in which the organic EL element substrate is bonded to the color filter substrate having the color conversion layer, it is easily inferred that a height of the separation wall on the color filter side and a height of the separation wall on the organic EL element substrate side are simply summed, which further increases an optical loss.
PTL 1: Japanese Patent Laid-Open No. 2006-32010
PTL 2: Japanese Patent Laid-Open No. 2008-78038
PTL 3: Japanese Patent Laid-Open No. 2005-93398